The functional cells of the blood are short-lived; they are replaced continuously by proliferation and differentiation of hematopoietic precursors. Since cell division is required, exposure to agents that destroy proliferative potential is followed by loss or reduction in blood-cell production, and often death. Ionizing radiation and certain chemotherapeutic drugs are examples of agents that are toxic to stem-cell proliferation. Death after irradiation can be prevented by transplanting hematopoietic cells from healthy donors. In the injured host, these are capable of initiating and maintaining the blood-cell production required for survival.